Note: Descriptions are shown in the official language in which they were submitted.
CA 02240376 1998-06-11
WO 97/23582 PCT/CA96/00862
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HYDROCRACKING OF HEADY HYDROCARBONS WITH CONTROL OF POLAR AROMATICS
Technical Field
This invention relates to the treatment of
hydrocarbon oils and, more particularly, to the
hydroconversion of heavy hydrocarbon oils in the
presence of additives, such as iron and/or coal
additives.
Background Art
Hydroconversion processes for the conversion of
heavy hydrocarbon oils to light and intermediate
naphthas of good quality for reforming feedstocks, fuel
oil and gas oil are well known. These heavy hydrocarbon
oils can be such materials as petroleum crude oil,
atmospheric tar bottoms products, vacuum tar bottoms
products, heavy cycle oils, shale oils, coal derived
liquids, crude oil residuum, topped crude oils and the
heavy bituminous oils extracted from oil sands. Of
particular interest are the oils extracted from oil
sands and which contain wide boiling range materials
from naphthas through kerosene, gas oil, pitch, etc.,
and which contain a large portion of material boiling
above 524°C equivalent atmospheric boiling point.
As the reserves of conventional crude oils decline,
these heavy oils must be upgraded to meet the demands.
In this upgrading, the heavier materials are converted
to lighter fractions and most of the sulphur, nitrogen
and metals must be removed.
This can be done either by a coking process, such
as delayed of fluidized coking, or by a hydrogen
addition process such as thermal or catalytic
hydrocracking. The distillate yield from the coking
process is typically about 80 wt~ and this process also
yields substantial amounts of coke as by-product.
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Work has also been done on an alternate processing
route involving hydrogen addition at high pressures and
temperatures and this has been found to be quite
promising. In-this process, hydrogen and heavy oil are
pumped upwardly through an empty tubular reactor in the
absence of any catalyst. It has been found that the
high molecular weight compounds hydrogenate and/or
hydrocrack into lower boiling ranges. Simultaneous
desulphurization, demetallization and denitrogenation
reactions take place. Reaction pressures up to 24 MPa
and temperatures up to 490°C have been employed.
Work has been done to develop additives which can
suppress coking reaction or can remove the coke from the
reactor. It has been shown in Ternan et al., Canadian
1.5 Patent No. 1,073,389, issued March 10, 1980 and
Ranganathan et al., United States Patent No. 4,214,977,
issued July 29, 1980, that the addition of coal or coal-
based additive results in the reduction of coke
deposition during hydrocracking. The coal additives act
as sites for the deposition of coke precursors and thus
provide a mechanism for their removal from the system.
Ternan et al., Canadian Patent No. 1,077,917
describes a process for the hydroconversion of a heavy
hydrocarbonaceous oil in the presence of a catalyst
prepared in situ from trace amounts of metals added to
the oil as oil soluble metal compounds.
In U.S. Patent No. 3,775,286, a process is
described for hydrogenating coal in which the coal was
either impregnated with hydrated iron oxide or dry
hydrated iron oxide powder was physically mixed with
powdered coal. Canadian Patent
No. 1,202,588 describes a process for hydrocracking
heavy oils in the presence of an additive in the form of
a dry mixture of coal and an iron salt, such as iron
sulphate.
Development of such additives has allowed the
reduction of reactor operating pressure without coking
CA 02240376 1998-06-11
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reaction. However the injection of large amounts of
'fine additive is costly, and the application is limited
by the incipient coking temperature, at which point
mesophase (a pre-coke material) is formed in increasing
amounts.
Further, it is shown in Jain et al., U.S. Patent
No. 4,969,988 that conversion can be further increased
through reduction of gas hold-up by injecting an anti-
foaming agent, preferab-1y into the top section of the
reactor.
Sears et al., U.S. Patent No. 5,374,348 teaches
recycle of heavy vacuum fractionator bottoms to the
reactor to reduce overall additive consumption by 400
more.
It is the object of the present invention to
provide a process for hydrocracking heavy hydrocarbon
oils using additive particles in the feedstock to
suppress coke formation in which improved yields can be
achieved by controlling the ratio of lower polarity
aromatics to asphaltenes in the reactor and thereby
inhibiting coke formation.
D~sclost~re of the Invention
According to the present invention, it has been
discovered that further improvements in the
hydroprocessing of heavy hydrocarbon oils containing
'additive particles to suppress coke formation are
achieved by adding aromatic oils, preferably in the form
of a recycled process derived heavy gas oil, to the
hydroprocessing feedstock such that a high ratio of
lower polarity aromatics to asphaltenes is maintained
during hydroprocessing and also recycling a downstream
fractionated heavy product (pitch) to the
hydroprocessing feedstock.
Thus, the present invention in one aspect relates
to a process for hydrocracking a heavy hydrocarbon oil
feedstock, a substantial portion of which boils above
524°C which comprises: passing a slurry feed of a
AMENDED SHEET
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mixture of heavy hydrocarbon oil feedstock and from
about 0.01-4.Oa by weight (based on fresh feedstock) of
coke-inhibiting additive particles comprising particles
of an iron compound having sizes less than aS~.m upwardly
through a confined vertical hydrocracking zone, said
hydrocracking zone being maintained at a temperature
between about 350° and 600°C a pressure of at least 3.S
MPa and a space velocity of up to 4 volumes of
hydrocarbon oil per hour per volume of hydrocracking
zone capacity. A mixed effluent containing a gaseous
phase comprising hydrogen and vaporous hydrocarbons and
a liquid phase comprising heavy hydrocarbons is removed
from the top of the hydrocracking zone and this mixed
effluent is passed into a hot separator vessel. A
gaseous stream comprising hydrogen and vaporous
hydrocarbons is withdrawn from the top of the separator,
while a liquid stream comprising heavy hydrocarbons and
particles of the coke-inhibiting additive is withdrawn
from the bottom. According to the novel feature, an
aromaticoi1 is added to the heavy hydrocarbon oil
feedstoc!c such that a high ratio of lower polarity
aromatics to asphaltenes is maintained during
hydroprocessing, this aromatic oil being a heavy gas oil
fraction obtained during fractionation of the liquid
bottom stream from the hot separator. Also, the iron
particles are used in the absence of an active
hydrogenation catalyst.
Preferably, the liquid stream from the bottom of
the separator is fractionated to obtain a heavy
hydrocarbon (pitch) stream boiling above 450°C,
preferably above 495°C, and containing the additive
particles, and a light oil product. At least part of
this fractionated pitch stream boiling above 450°C and
containing additive particles is recycled to form part
of the heavy hydrocarbon oil feedstock.
The process of this invention is capable of
processing a wide range of heavy hydrocarbon feedstocks.
A~tIENDE~ SI-~EE~
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Thus, it can process aromatic feedstocks, as well as
feedstocks which have traditionally been very difficult
to hydroprocess, e.g. visbroken vacuum residue,
deasphalted bottom materials, off-specification asphalt,
grunge from the bottom of oil storage tanks, etc. These
difficult-to-process feedstocks are characterized by low
reactivity in visbreaking, high coking tendency, poor
conversion in hydrocracking and difficulties in
distillation. They have, in general, a low ratio of
polar aromatics to asphaltenes and poor reactivity in
hydrocracking relative to aromatic feedstocks.
Most feedstocks contain asphaltenes to a more or
less degree. Asphaltenes are high molecular weight
compounds containing heteroatoms which impart polarity.
It has been shown by the model of Pfeiffer and Sal,
Phys. Chem. as 139 (19x0), that asphaltenes are
surrounded by a layer of resins, or polar aromatics
which stabilize them in colloidal suspension. In the
absence of polar aromatics, or if polar aromatics are
diluted by paraffinic molecules, these asphaltenes can
self-associate, or flocculate to form larger molecules
which can precipitate out of solution. This is the
first step in coking.
In a normal hydrocracking process; there is a
tendency for asphaltenes to be converted to lighter
materials, such as paraffins and aromatics. Polar
aromatics are also converted to lighter materials, but
at a higher rate than the asphaltenes. The result is
that the ratio of polar aromatics to asphaltenes
decreases, and the ratio of paraffins to aromatics
increases as the reaction progresses. This eventually
leads to asphaltene flocculation, mesophase formation
and coking. This coking can be minimized by the use of
an additive, and coking can also be controlled at the
incipient coking temperature, which is the temperature
at which coking just begins for a fixed additive
concentration. This temperature is quite low for poor
~n ~~~r
,', :L.,jC~:_.:
CA 02240376 1998-06-11
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feeds, resulting in poor conversion.
In the process of this invention, it is now
possible to very successfully process feedstocks that
are traditionally very difficult to process. This is
achieved by firstly recycling the fractionated pitch
stream boiling above 45~°C with additive particles and
secondly adding a lower polarity aromatic oil to the
feedstock, the aromatic oil being in the form of a
recycle of heavy gas oil from the hydrocracker itself.
As stated above, the asphaltenes in the feedstock
are surrounded by a shell of highly polar aromatics
which are a problem in terms of coke formation.
Increasing conversion increases the polarity of the
aromatic shell around the asphaltene. However, in
accordance with this invention, by introducing lower
polarity aromatics into the reaction system, these lower
polarity aromatics are able to surround and mix with and
dilute the highly polar aromatics. This also tends to
reduce the polar gradient so as to allow hydrogen to
pass in through the shell and to allow olefinic
fragments to diffuse out and prevent recombination.
This permits time for the asphaltene to break down in
the process. The term "aromatics of lower polarity" as
usedyherein means aromatic oils of low polarity relative
to the polarity of components such as asphaltenes in the
heavy hydrocarbon feedstock.
Thus, by controlling the highly polar aromatics in
the reaction system according to this invention, a
balance is maintained such that the asphaltenes "see"
aromatics including those of lower polarity everywhere.
Paraffins that are formed are diluted and can diffuse
quickly in this continuum. Also as explained above, any
mass transfer limitations that were previously caused by
the highly polar aromatic shell are minimized and the
dispersion of olefins in the aromatics of lower polarity
lessens recombination reactions and decreases the
probability of recombination with the asphaltenes. Non-
Lei
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aromatic fragments formed from asphaltenes diffuse away
from the asphaltene core and prevent molecular weight
growth through recombination.
-~ a n
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By controlling polar aromatics through further
aromatics addition, pitch reactivity is maintained and
coking tendency is reduced. Pitch can be recycled under
these conditions, which results in a conversion
increase. This reduces pitch molecular weight which
- further stabilizes the operation at high overall
conversion. It was expected that this extensive
recycling would have a serious effect on the
productivity of the reactor, but it was discovered that
this effect on productivity is more than offset by the
higher reactor temperatures that became possible. It
appears that there are no compounds that intrinsically
form coke, only limitations imposed by the colloidal
system, and by mass transfer in the system. It further
appears that there is no intrinsic incipient coking
temperature for each feedstock, only the necessity to
suspend the additive, and suspend and carry asphaltenes
until they are converted or exit the reactor.
There is an additional benefit of high conversion
that is not immediately apparent. The liquid traffic in
the reactor, which is made up of pitch and low polar
aromatic oil, is much reduced. This can be controlled
by recycle, and in such a way that the reactor additive
is much increased over a once through operation. This
allows the process to be much more stable as incremental
additive surface area is available to aid hydrogen
transfer to the olefins and aromatics generated.
Best Modes for Carrying Out the Invention
The process of this invention can be operated at
quite moderate pressure, preferably in the range of 3.5
to 24 MPa, without coke formation in the hydrocracking
zone. The reactor temperature is typically in the range
of 350° to 600°C with a temperature of 400° to
500°C
being preferred. The LHSV is typically below 4 h-1 on a
fresh feed basis, with a range of 0.1 to 3 h-1 being
preferred and a range of 0.3 to 1 h-1 being particularly
preferred.
~"~,. ~ k : ~'~ 1.~ ~n ~t (.; ~ 1 ( i ° i w E w ~.~'..~"
CA 02240376 1998-06-11
a
An important advantage of this invention is that
the process can be operated at a higher temperature and
lower hydrogen partial pressure than usual processes for
cracking heavy oils. This higher temperature provides a
better balance between the thermal asphaltene
decomposition and the aromatic saturation and thermal.
decomposition. Lower hydrogen partial pressures lead to
efficiencies in hydrogen management and reduced capital
and operating costs of the equipment.
Although the hydrocracking can be carried out in a
variety of known reactors of either up or downflow, it
is particularly well suited to a tubular reactor through
which feed and gas move upwardly. The effluent from the
top is separated in a hot separator and the gaseous
stream from the hot separator can be fed to a low
temperature, high pressure separator where it is
separated into a gaseous stream containing hydrogen and
less amounts of gaseous hydrocarbons and liquid product
stream containing light oil product.
A variety of added particles can be used in the
process of the invention, provided these particles are
able to survive the hydrocracking process and remain
effective as part of the recycle. Particularly useful
additive particles are those described in Belinko et
al., U.S. Patent No. a,963,2a7, issued October 16, 1990.
Thus, the particles are typically an iron compound,
preferably ferrous sulfate having particle sizes less
than 45 um and with a major portion, i.e. at least 500
by weight, preferably having particle sizes of less than
10 Vim.
According to a preferred embodiment, the particles
of iron sulphate are mixed with a heavy hydrocarbon oil
feed and pumped along with hydrogen through a vertical
reactor. The liquid-gas mixture from the top of the
hydrocracking zone can be separated in a number of
different ways. One possibility is to separate the
liquid-gas mixture in a hot separator kept at a
CA 02240376 1998-06-11
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temperature in the range of about 200°-470°C and at the
pressure of the hydrocracking reaction. A portion of
the heavy hydrocarbon oil product from the hot separator
is used to form the recycle stream of the present
invention after secondary treatment. Thus, the portion
of the heavy hydrocarbon oil product from the hot
separator being used for recycle is fractionated in a
distillation column with a heavy liquid or pitch stream
being obtained. This pitch stream boils above 495°C
with a pitch boiling above 524°C being particularly
preferred. This pitch stream is then recycled back to
form part of the feed slurry to the hydrocracking zone.
Part of this pitch stream may also comprise a pitch
product and may be fed to a thermal cracking process.
A~-~. aromati c gas oil fraction preferably boiling above
400°C is also removed from the distillation column and
it is recycled back to form part of the feedstock to the
hydrocracking zone for the purpose of controlling the
ratio of polar aromatics to asphaltenes.
Preferably the recycled heavy oil stream makes up
in the range of about 5 to 15 o by weighs of the
feedstock to the hydrocracking zone, while the aromatic
oil, e.g. recycled aromatic gas oil, makes up in the
range of 15 to 50 o by weight of the feeastock,
depending upon the feedstock structures.
The gaseous stream from the hot separator
containing a mixture of hydrocarbon gases and hydrogen
is further cooled and separated in a low temperature-
high pressure separator. By using this type of
separator, the outlet gaseous stream obtained contains
mostly hydrogen with some impurities such as hydrogen
sulphide and light hydrocarbon gases. This gaseous
stream is passed through a scrubber and the scrubbed
hydrogen may be recycled as part of the hydrogen feed to
the hydrocracking process. The hydrogen gas purity is
maintained by adjusting scrubbing conditions and by
adding make up hydrogen.
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The liquid stream from the low temperature-high
pressure separator represents a light hydrocarbon oil
product of the present invention and can be sent for
secondary treatment.
According to an alternative embodiment, the heavy
oil product from the hot separator is fractionated into
a top Light oil stream and a bottom stream comprising
pitch and heavy gas oil. A portion of this mixed
bottoms stream is recycled back as part of the feedstock
to the hydrocracker while the remainder of the bottoms
stream is further separated into a gas oil stream and a
pitch product. The gas oil stream is then recycled to
be feedstock to the hydrocracker as additional low polar
aromatic stock for polar aromatic control in the system.
l5 The process of the invention can convert heavy gas
oil to extinction and can also convert~a very high
proportion of the heavy hydrocarbon materials of the
feedstock to liquid products boiling below 400°C. These
features make the process useful as an outlet for
surplus refinery aromatic streams. It is also uniquely
useful as an outlet for junk feedstocks. Furthermore,
the process represents a unique method of control for
the hydrocracking of heavy hydrocarbon oils by
controlling the quantities and compositions of the pitch
stream and the aromatic oil stream fed as part of the
feedstock to the hydrocracking process.
For some feedstocks, it has been found to be
advantageous to conduct a treatment prior to
hydrocracking to remove high boiling paraffinic
material.
Brief Description of the DrawincLs
,
For a better understanding of the invention,
reference is made to the accompanying drawings in which: .
Fig. 1 is a schematic flow sheet showing a typical
hydrocracking process to which the present invention may
be applied;
~. A
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Fig. 2 is a plot of hydrogen in pitch vs.
conversion;
Fig. 3 is a plot of nitrogen in pitch vs.
conversion;
Fig. 4 is a plot of asphaltene in pitch vs.
conversion;
Fig. 5 is a plot of asphaltene in reactor products
vs. conversion;
Fig. 6 is a plot of pitch quality vs. VGO recycle
rate;
Fig. 7 is a plot of yield shift with VGO recycle;
Fig. 8 is a plot of pitch conversion vs. pitch LHSV;
Fig. 9 is a plot of TIOR/additive vs. reactor
additive concentration;
Fig. 10 is a plot of coke yield vs. HVGO recycle;
Fig. 11 is a plot of additive coke vs. pitch
molecular weight; and
Fig. 12 is a plot of quaternary carbon vs. polar
aromatic phase/total aromatic phase.
Description of the Preferred Embodiments
In the hydrocracking process as shown in Figure l,
the iron salt additive is mixed together with a heavy
hydrocarbon oil feed in a feed tank 10 to form a slurry.
This slurry, including heavy oil or pitch recycle 39, is
pumped via feed pump 11 through an inlet line 12 into the
bottom of an empty reactor 13. Recycled hydrogen and make
up hydrogen from line 30 are simultaneously fed into the
reactor through line 12. A gas-liquid mixture is
withdrawn from the top of the reactor through line 14 and
introduced into a hot separator 15. In the hot separator
the effluent from reactor 13 is separated into a gaseous
stream 18 and a liquid stream 16. The liquid stream 16
is in the form of heavy oil which is collected at 17.
The gaseous stream from hot separator 15 is carried
by way of line 18 into a high pressure-low temperature
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separator 19. Within this separator the product is
separated into a gaseous stream rich in hydrogen which
is drawn off through line 22 and an oil product which is
drawn off through line 20 and collected at 21.
The hydrogen-rich stream 22 is passed through a
packed scrubbing tower 23 where it is scrubbed by means
of a scrubbing liquid 24 which is recycled through the
tower by means of a pump 25 and recycle loop 26. The
scrubbed hydrogen-rich stream emerges from the scrubber
via line 27 and is combined with fresh make-up hydrogen
added through line 28 and recycled through recycle gas
pump 29 and line 30 back to reactor 13.
The heavy oil collected at 17 is used to provide
the heavy oil recycle of the invention and before being
recycled back into the slurry feed, a portion is drawn
off via line 35 and is fed into fractionator 36 with a
bottom heavy oil stream boiling above 450°C, preferably
above 524°C being drawn off via line 39. This line
connects to feed pump 11 to comprise part of the slurry
feed to reactor vessel 13. Part of the heavy oil
withdrawn from the bottom of fractionator 36 may also be
collected as a pitch product 40.
The fractionator 36 may also serve as a source of
the aromatic oil to be included in the feedstock to
reactor vessel 13. Thus, an aromatic heavy gas oil
fraction 37 is removed from fractionator 36 and is feed
into the inlet line 12 to the bottom of reactor 13.
This heavy gas oil stream preferably boils above 400°C.
A light oil stream 38 is also withdrawn from the top of
fractionator 36 and forms part of the light oil product
21 of the invention.
Description of the Preferred Embodiments ,
Certain preferred embodiments of this invention are
illustrates by the following non-limiting Examples.
Exam~ale 1 (Comparative)
Tests were carried out on a hydrocracker pilot
plant of the type shown in Figure 1 using as feedstock
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Cold Lake Vacuum Bottoms (CLUB), with 5.6% sulphur, 75%
wt of 524°C+ material and 5° API. First the CLVB was
tested in a once-through mode, and a model developed for
' this operation and a range of conditions. Next, the
pilot plant was operated with pitch recycle, and it was
found that the rate constant for the recycled material
was:
K = 0.953 - 0.0083 (524°C+ Conversion)
where conversion is in weight percent. Thus the rate
l0 constant for fresh feed would be K = 0.953, and for
pitch product from an 80% of 524°C conversion operation
it would be K = 0.953 - 0.0083 (80) - 0.289. This is a
significant drop in reactivity for the following typical
pilot plant conditions:
Temperature 447°C Feed 80% fresh/20% recycle
Pressure 13.8 MPa Recycle cut point 480°C
Gas Rate 28 L/min Fresh feed LHSV 0.48
Gas Purity 85% H2 Additive* 1.2% on total
feed
Reactor 2.54 cm ID by 222 cm high
*The additive used was ferrous sulfate having particle
sizes less than 45 ~.m as described in U.S. Patent No.
4,963,247.
This showed that recycled pitch was less reactive
than fresh feed, and that its reactivity was dependent
on the conversion (reaction severity) to which it was
subjected. This data discouraged recycle of pitch for
canversion reasons, and seemed to show that there was a
portion of the feed which was inherently not
convertible, or convertible only with difficulty.
These tests did, however, show that recycled iron
sulphide additive retained its activity, which is a
strong incentive for recycle of pitch (recycle reduced
fresh additive requirement by as much as 40% in the
study) .
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Example 2 (Comparative)
Visbroken vacuum residue from a commercial
visbreaker in the Montreal refinery of Petro-Canada (a
Shell soaker type) was tested in the same pilot plant as
in Example 1. Conditions for a sample test were as
follows:
Temperature 449°C
Pressure 13.8 MPa
Gas Rate 28 L/min
Gas Purity 85o HZ
Fresh Feed LHSV 0.5, feed origin - Venezuelan Blend
24
Additive* 3°s on total feed
*The additive used was ferrous sulfate having particle
sizes less than 45 ~.m as described in U.S. Patent No.
4,963,247.
Pitch conversion was found to be 83%, and this was
comparable to 85% conversion obtained with Blend 24
vacuum bottoms feed under similar conditions. This run
showed that a visbroken material could be run at
comparable conversion to virgin material of same boiling
range. However it also showed that pitch quality
deteriorates with respect to hydrogen and nitrogen
content (Figures 2 and 3), and that asphaltene content
increases in pitch as conversion increases (Figure 4).
In Figures 2, 3 and 4, Feed A was a Cold Lake residuum
and Feed B was a visbroken vacuum residuum derived from
Venezuelan Blend 24. The curves for Cold Lake residuum
show that there are similar changes in pitch properties
when a virgin material is hydrocracked. For both
feedstocks there was a uniform destruction of feed
asphaltenes (Figure 5) and a deterioration in pitch
properties mentioned above. Decreases in pitch hydrogen
content indicate condensed aromatic ring structures, and .
increased nitrogen indicates that these ring atrur_t-_"rP~
are more polar. These changes are very significant and
are believed to be irreversible for the above systems.
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Exam~~le 3
Examples 1 and 2 were both run without feeding
extra aromatic oil to the hydrocracker. This example
' shows the effects of adding extra aromatic oil in the
form of vacuum gas oil (VGO).
Feedstock in this case was Cold Lake residuum of
5.5 API, sulphur 5.0% , nitrogen 0.6% and 15% boiling
below 524C. This material was obtained from a refinery
run and contained up to 20% of Western Canadian blend.
The gas oil obtained from a once-through run with this
feedstock at 86% conversion, was at 14.9% API, 2.2%
sulphur, 0.53% nitrogen and had 10%, 50% and 90% points
of 330, 417, and 497C respectively. Tests were made
which simulate 30, 50, 75 and 100% recycle of the gas
oil produced on a once-through basis corresponding to
8.5, 14.1, 19.5 and 24.5 wt% of fresh feed respectively
in Figures 6 - 8. All runs were at 3.6% iron-sulfate
additive as described in Example 2 on the vacuum tower
bottoms portion of the feed.
From Figure & it can be seen that, at Constant
conversion, pitch quality increased with increasing gas
oil recycle. Hydrogen content increased by a full 1% to
8% when gas oil was recycled "to extinction".
Furthermore, nitrogen content decreased from 240 to 200%
in the pitch relative to the fresh feed.
Figure 7 shows that the gas oil has been converted
to lighter products, an additional plus feature for this
operation as gas oil can be converted to near
extinction. All tests were done with 3.6% additive on
fresh feed, which probably masked any effect of VGO
recycle on coke yield. This will be discussed further
in Example 4. Figure 8 shows that there was little
capacity lost with added VGO recycle, in the amount of
8.8, 14.5, 20.1 and 25.2 wt% based on fresh feed. This
is a surprising result as there is some VGO accumulation
in the reactor, which would be increased under VGO
recycle conditions and which would tend to decrease
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conversion. Pilot plant testing confirmed that VGO
conversion is significantly accelerated with increasing
temperature.
The above results show that:
1. An improvement in pitch quality is obtained at
constant conversion when vacuum gas oil is recycled
to the reactor.
2. The VGO is cracked significantly to lighter
products when recycled.
Examt~le 4
This example gives data from commercial operation
of a nominal 5000 BPD hydrocracking unit. The reactor
in this case was 2 m in diameter by 21.3 m high.
Conditions for a run with aromatics addition and pitch
recycle were as follows:
Liquid Charge:
Fresh feed' 3218 BPD, 8.5° API
Aromatics addition 823 BPD
Recycle of Pitch 652 BPD
Total Feed 4693 BPD
Unit Temperature 464°C
Unit Pressure 13.9 MPa (2024 psi?
Recycle Gas Purity 750
524°C+ Conversion 92~ wt
HZ Uptake 907 SCFB
Additive Rate -- wt% on feed
2.3 fresh as FeS04 ~ Hz0
2.6 recycled as FeS04 ~ HZO
Additive in Reactor 9.5 wto
TIOR in Reactor 1.86 wto as FeS
"Fresh feed was visbreaker vacuum tower bottoms from
Flotta crude.
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Product slate was as follows:
Fuel Gas 14.2% vol on fresh feed
IBP-204C 23.9% vol on fresh feed
204-343C 37.9 vol on fresh feed
343-524C 36.9% vol on fresh feed
524C+ 5.2~ vol on fresh feed
The above are typical conditions for the
combination of pitch recycle and aromatics addition to
control polar aromatics in the system for increased
efficiency. Without pitch recycle and aromatics
addition the expected conversion at this fresh feed
charge rate would be 65 to 70%, limited by the incipient
coking temperature for this feedstock at about 440°C.
There is obvious improvement over a once-through
operation, and over a pitch recycle operation without
addition of supplementary polar aromatics. This
improvement is not only in conversion, but in additive
utilization as shown in Figure 9, a plot of
coke/additive ratio in the reactor versus additive
concentration in the reactor. Historical "once-through"
numbers for reactor additives are in the 1-2% range.
Now ~niith pitch recycle and aromatic addition these have
increased to 5 - 9 wt% range due to increased
conversion, concurrent product vaporization, and to
additive returned with the pitch.
The increased reactor additive concentration
results in lower coke on additive and to conditions for
improved conversion, including increased hydrogen
addition to pitch which reduces the slide in pitch
quality, rendering all pitch capable of conversion.
Coke (TIOR) yield can also be reduced by recycling VGO
produced in the unit itself, as shown in Figure 10 which
gives the effect of VGO recycle (as a o of fresh feed)
on coke yield. The additive was used in amounts of 1.2,
2.3 and 3.0 wt% based on fresh feed. The effect is
smaller when additive is plentiful, becomes more
CA 02240376 1998-06-11
WO 97/23582 PCT/CA96/00862
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significant at low feed additive levels, and very
dramatic at 1.2% additive on fresh feed.
Example 5
This example gives aromatics analyses for selected
streams in support of the understanding that polar
aromatics control is the key to high conversion and
reduced additive consumption.
Figure Z1 gives average pitch molecular weight
versus coke (TIOR) in the reactor. The increased
average aromatic carbon content of the reactor contents
as shown by the lines allows for operating an elevated
coke in the reactor. In all the commercial examples in
Figure 11, the mesophase coke levels were much less than
5 microns. The increased stability afforded by the
aromatic oil allows for higher reactor operating
temperatures which allows for maintaining the average
molecular weight of the pitch low enough for coking
control even with extremely difficult to convert
feedstock.
Table 1 gives hydrocarbon type analyses for
aromatic oil (in this~case slurry oil or decant oil from
a Fluid Catalytic Cracker), and for other feeds and
products mentioned in the above Examples. The process
generated VGO and decant oil are clearly similar. These
samples were taken during a run in which the commercial
plant of Example 4 was operating with a visbreaker
vacuum tower bottoms feed, with pitch recycle and slurry
oil addition similar to Example 4.
Table 1 shows that the ratio of the aromatic and
polar aromatics relative to the nC., insolvable
asphaltenes is reduced in both the reactor content and
the unconverted pitch relative to the feed. The ratio
of the aromatics + polar aromatics to asphaltene in the
VVR feed is about 3.86. This ratio drops as the feed
is converted with the ratio in the unconverted pitch
dropping to 2.07.
CA 02240376 1998-06-11
Vt~O 97/23582 PCT/CA96/00862
- 19 -
For VGO and aromatic oil, the di, tri and tetra-
aromatics are predominant, and the streams seem to be
interchangeable. An aromatics breakdown for different
feedstocks and products is shown in Table 2.
Table 3 shows an elemental analysis of the reactor
feed, reactor sample and the unconverted pitch. The
visbreaker vacuum tower bottoms (polar phase) is very
low in hydrogen content at about 8.2 wt% and has a very
high nitrogen content of 1.1 wt~. The hydrogen content
of the saturate phase is significantly higher at 13.8
wt%. The nC., solvent portion of the WR feed has a
hydrogen content of about 10.2 wt% and a nitrogen
content of about 0.43 wto.
The reactor contents and the unconverted pitch are
found to have similar composition. The nitrogen content
of the polar aromatic phase is shown to have been
elevated in both the reactor contents and the
unconverted pitch relative to the fresh feed. The
nitrogen content of the aromatic fraction of the reactor
contents and the unconverted pitch is found to be about
the same as the fresh feed. The combination of the data
in Table 1 and Table 3 shows the nitrogen content of the
polar aromatics is concentrating at the same time that
the relative amount of polar aromatics to asphaltenes is
decreasing.
Table 4 shows the aromatic carbon distribution in
the polar aromatic, aromatic and saturate fractions of
the feed, reactor and unconverted pitch. The
aromaticity of the aromatic and polar aromatic phases
have increased significantly relative to the feed.
However, the quaternary carbons as a ratio to the total
aromatic carbons has been reduced. The quaternary
carbons in the VVR fresh feed made up 49 percent of the
aromatic carbons in the aromatic and polar aromatic
phases. This was reduced to 43 percent of the aromatic
carbons in the unconverted pitch, aromatic and polar
aromatic phases.
CA 02240376 1998-06-11
WO 97/23582 PCT/CA96/00862
- 20 -
Figure 12 is a plot showing the relationship of the
quantity of quaternary carbon present in the aromatic
and polar aromatic phases with the ratio of the polar
aromatics phase to the combined polar aromatic and
aromatic phases.
The data presented in the above examples shows that
the aromatics. surrounding the asphaltenes are converted
at a faster rate relative to the asphaltenes. If the
aromatics phase is kept in balance with the asphaltenes,
and the polar strength of the polar aromatic phase is
limited by dilution by less polar aromatics, then
mesophase generation tendency can be controlled and the
high conversion of very hard to process feedstocks can
be achieved.
CA 02240376 1998-06-11
WO 97/23582 PCT/CA96/00862
-21-
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CA 02240376 1998-06-11
WO 97/23582 P'CT/CA9b/00862
- 22 -
Table 2
By Weight
Mono-di- tri- tetra- Penta-~-
Aroma tics Aromatics Aromatics Aromatics
Aromatics
Naphtha 15 -- __ __ __
Distillate 27 16 -- __ __
Lt. VGO 20 37 5 -- __
VGO 4 22 25 10 --
Aromatic 2 23 30 g __
oil
Feed VVR 9 8 7 3 12'
Pitch 2 8 5 6 12'
'Has been deasphalted.
Table 3
ELEMENTAL ANALYSIS OF PETROLEUM FRACTIONS
Elemental (wt
Fraction Sample
Carbon Hydrogen Nitrogen
Feed VVR. 85.0 8.2 1.1
Polars Reactor Middle 87.0 6.5 2.0
Pitch 86.8 6.5 1.8
Feed WR 86.4 9.5 0.3
Aromatics Reactor Middle 89.6 6.8 0.3
Pitch 89.3 6.8 0.2
Feed WR 86.0 13.8 0.0
Saturates Reactor Middle 86.0 14.0 0.0
Pitch 86.0 13.8 0.0
CA 02240376 1998-06-11
WO 97/2582 PCT/CA96/00862
- 23
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